This invention relates to dual wall, air gap, engine exhaust duct components, a blank therefor, and a method of making such.
When exhaust gases of an internal combustion engine are conducted through the ducts of the metal exhaust manifold and connected exhaust ducts such as a crossover pipe, to the catalytic converter, it is desirable to lose minimal heat from the gases upstream of the catalytic converter. This keeps them as hot as possible for the quickest "light off" in the catalytic converter, to minimize unwanted emissions. It also minimizes temperature rise in the engine compartment. It is recognized in the industry that the use of a double wall construction with an air gap therebetween over most of the length thereof is advantageous for achieving lower heat transfer. This type of technology is generally shown and/or described, for example, in U.S. Pat. Nos. 4,619,292; 4,185,463 and 4,022,019. Known methods for the manufacture of dual wall, air gap, exhaust gas, duct components are complex and costly, however, with varying techniques having been proposed, such as splitting the outer tube and welding the split outer pipe components around the inner tube as in U.S. Pat. Nos. 4,619,292; 4,656,712 and 4,501,302; or welding an assembly around the inner tube as in U.S. Pat. No. 4,142,366.
Another known method used commercially for making exhaust system components with air gap characteristics utilizes the technique of placing one tube inside another tube while leaving the desired air gap, then bending and forming them to the desired overall shape with a medium such as sand, lead, shot, ice, or the like placed between the outer tube and the inner tube in an effort to control the gap between the two during the bending and forming operations. Unfortunately, most media inserted in this fashion do not react to bending and forming forces in the same way that the metal in the tubes would. It is also very difficult to control the hoped-for gap between the two components when bending and forming. Exhaust duct components are often of peculiar configuration and complex in nature, with enlargements or protrusions in some areas, recesses in other area to accommodate the engine compartment, etc., bends along their length to extend in the desired direction, and the like. Attempting to provide a dual wall structure with the desired air gap for these complexly configurated components of the exhaust system presents significant practical and economical difficulties.